1. Field of the Invention
The present invention relates to an image forming device, and in particular, relates to a scanning optical unit provided in an image forming device, which scanning optical unit forms an image on a recording medium by scanning, in a main scanning direction, a laser beam which is diffracted by an acousto-optic modulating element (an acousto-optic modulator).
2. Description of the Related Art
An image exposure device, which scans a laser beam to write an image onto a photographic printing paper, is used in a digital laboratory system. The image exposure device is provided with light sources which emit laser beams of the colors of R, G, B. On the basis of color image data, the laser beams emitted from the light sources of the respective colors are modulated, and are illuminated onto a deflector such as a polygon mirror. The laser beam from the deflector is deflected in the main scanning direction and is illuminated onto a photographic printing paper.
The scanning optical unit provided at the image exposure device is provided with a light source unit comprising LDs of the colors of R, G, B, a collimator lens for condensing light, an acousto-optic modulator (AOM), an f.theta. lens, a cylindrical lens, and the like. The laser beams of R, G, B illuminated from the light source unit are illuminated onto a predetermined point of the polygon mirror, and main scanning is carried out by the laser beam reflected at the polygon mirror being illuminated onto a photographic printing paper.
In a scanning optical unit structured in this manner, the illuminated position of the laser beam is detected by a sensor, and the time at which the main scan starts is determined. The laser beam transmitted through the AOM is diffracted in accordance with the determined time at which the main scan starts.
The sensor that detects the laser beam may be structured so as to receive light which is the laser beam which has been reflected by the polygon mirror and further reflected by a reflecting mirror (Refer to Japanese Patent Application Laid-Open (JP-A) No. 6-202016).
When the laser beam is illuminated onto a peripheral edge portion of this reflecting mirror, flare reflection may occur, and the photographic printing paper may be needlessly exposed due to the flare-reflected laser beam. When the photographic printing paper is needlessly exposed by such flare reflection, the quality of the finish of the recorded image deteriorates.
Attempts have been made to prevent flare reflection in a main scanning optical unit by changing the intensity of light or the direction of diffraction of the light diffracted by the AOM which detects the laser beam by a sensor, such that the laser beam is not illuminated onto a peripheral portion of the reflecting mirror.
However, in the AOM, the laser beam passing through the acousto-optic modulating medium is diffracted by ultrasonic waves which are generated in the acousto-optic modulating medium by the transducer. Therefore, there is a time difference between the time when the ultrasonic waves are generated by the transducer and the time the laser beam is diffracted.
As a result, during the period of time from after the laser beam is detected by the sensor until the AOM is operated, the laser beam may reach a peripheral portion of the reflecting mirror, and flare reflection cannot always be reliably prevented.